Threshold phenomena in erosion driven by subsurface flow

TL;DR

This study investigates how subsurface groundwater flow influences erosion and slope stability, revealing that subsurface pressures lower the threshold for sediment mobilization and promote channelization at slopes steeper than a critical angle.

Contribution

It provides a quantitative analysis of how subsurface flows modify erosion thresholds and induce channelization, extending the classical Shields criterion to include subsurface hydrodynamic stresses.

Findings

Subsurface flow reduces the erosion threshold at slopes above a critical angle.

Slopes steeper than this critical angle are inherently unstable to channelization.

Subsurface pressures significantly influence sediment mobilization modes.

Abstract

We study channelization and slope destabilization driven by subsurface (groundwater) flow in a laboratory experiment. The pressure of the water entering the sandpile from below as well as the slope of the sandpile are varied. We present quantitative understanding of the three modes of sediment mobilization in this experiment: surface erosion, fluidization, and slumping. The onset of erosion is controlled not only by shear stresses caused by surfical flows, but also hydrodynamic stresses deriving from subsurface flows. These additional forces require modification of the critical Shields criterion. Whereas surface flows alone can mobilize surface grains only when the water flux exceeds a threshold, subsurface flows cause this threshold to vanish at slopes steeper than a critical angle substantially smaller than the maximum angle of stability. Slopes above this critical angle are unstable to channelization by any amount of fluid reaching the surface.